The present invention relates to a photo-electric converter having a function of correcting an offset voltage in a photo-electric converting circuit thereof, which converts a light amount into an electric signal and is used for measuring luminous intensity and the like.
Such a kind of photo-electric converter has a photo-electric converting circuit for converting a photocurrent from a photosensitive element into a voltage or an electric current. One of the error components contained in an output from the photo-electric converting circuit is an offset voltage.
A method for correcting such an offset voltage is conventionally known which comprises putting a cap or the like on the front face of a photosensitive element to shade the same, storing the output from the photo-electric converting circuit at that time as a digital value in a CPU, and subtracting by means of the CPU the abovementioned stored digital value from a digital value representing a light measurement value (see, for example, U.S. Pat. No. 4,201,472).
Now, an offset voltage correction in a photo-electric converting circuit will be described below in detail with reference to the appended drawings,
FIG. 9 shows an example of a conventional photo-electric converting circuit of a linear conversion type. The output voltage V.sub.out is represented by the following formula in connection with a photocurrent I.sub.P generated from a photodiode (photosensitive element) SP.sub.1 when a light is thrown on the photodiode SP.sub.1, a bias electric current I.sub.B and an offset voltage V.sub.off in an operational amplifier (photo-electric converting circuit) IC.sub.1. EQU V.sub.out =-(I.sub.P -I.sub.B)R.sub.f +V.sub.off
The signal component here is EQU -I.sub.P .multidot.R.sub.f
and the error component is EQU I.sub.B .multidot.R.sub.f +V.sub.off
The operational amplifier IC.sub.1 generally has a terminal for controlling the offset voltage to a zero value, and it is possible to make V.sub.off .perspectiveto.0 by means of an outer volume controller VR.sub.1. However, the offset voltage depends on both temperature and power supply voltage. Accordingly, if the offset voltage is controlled to be zero in one condition, it cannot be kept zero in another condition, for example, when the temperature changes, and it must be controlled by means of the outer volume controller each time.
Further, it is impossible to eliminate the bias electric current I.sub.B to zero, and this bias electric current I.sub.B becomes a large error component in the case of measuring the photocurrent I.sub.P to a very small amount. Similarly to the offset voltage, the bias electric current depends on temperature and power supply voltage.
The examples of conventional arrangements for correcting an error caused by a offset voltage and a bias electric current are shown in FIGS. 10 and 11.
Referring now to FIG. 10, firstly a movable shading plate D is put in front of a photodiode SP.sub.1 to keep the photodiode SP.sub.1 from the light. At this time, the photocurrent is zero, and the output voltage V.sub.out is as follows. EQU V.sub.out =+I.sub.B .multidot.R.sub.f +V.sub.off
This voltage is analogue-to-digital (hereinafter referred to as A-D) converted by means of an A-D converter 101 to obtain a corresponding digital value N.sub.o, which is read and stored in a CPU 102.
Nextly, when the shading plate is moved and the light is thrown on the photodiode SP.sub.1, a photocurrent I.sub.P flows and the output voltage V.sub.out becomes as follows. EQU V.sub.out =-(I.sub.P -I.sub.B)R.sub.f +V.sub.off
This voltage is A-D converted to obtain a corresponding digital value N'. The CPU 102 reads the digital value N' and executes the following subtraction to correct the error. EQU N=N'-N.sub.o
Here, N is a digital value corresponding to -IP.multidot.Rf and is displayed as a light measurement value by a display 103.
The abovementioned arrangement is disadvantageous in that a mechanism for moving the shading plate D is required and the arrangement becomes complicated and expensive.
Referring now to FIG. 11, a chopper CH to be rotatably or vibratorily driven by a motor M or the like is provided in front of a photodiode SP.sub.1 so that an incident light on the photodiode SP.sub.1 is converted into an AC current by the chopper CH and processed. And the output voltage V.sub.out is fed through a capacitor C to an amplifier 104, amplified by the amplifier 104, rectified by a rectifier 105 and displayed by a display 103.
This arrangement is disadvantageous in that a chopper and corresponding mechanism and a circuit for rotating or vibrating the chopper are required, and that when a light source such as a fluorescent light is used, the frequency of the chopper must be changed in accordance with the commercial frequency of the incident light.